Gas turbine engines have a combustion chamber wherein a fuel is introduced and mixed with an oxygen-containing fluid (an oxidizer, typically air), generating a mixture that is combusted, to generate hot gases that are expanded in a turbine.
In particular, the combustion chamber has mixing devices connected to a combustion device; the fuel is introduced into the mixing devices such that as it passes through it, it mixes with the oxygen containing fluid and increases its temperature; then when the fuel enters the combustion device, it burns.
The described operation mode requires that the reactivity conditions be comprised in a correct window, such that combustion neither starts too early (where it would cause so called flashback, i.e. combustion in the mixing devices) nor too late.
Reactivity conditions depend on a number of factors and, in particular, on the fuel temperature and oxygen concentration of the environment housing the fuel; in particular, reactivity increases (meaning that reactions in the combustion process accelerate) with increasing of the fuel temperature and oxygen concentration, whereas it decreases with decreasing of fuel temperature and oxygen concentration.
In some cases the gas turbine engine may operate at actual reactivity conditions that are different (in particular lower) from the design reactivity conditions.
Operation with fuel at reduced reactivity conditions may for example occur at part load (since the temperature of the flame is lower than the flame temperature at full load) or in case the external temperature is very low (external temperature influences the temperature within the combustion chamber) or in case the oxygen concentration is low (for example when the gas turbine engine operates together with a flue gas recirculation system).
When operating under reduced reactivity conditions, the flame operation is close to extinction and typically, because of non-uniformities in fuel or air distribution, some mixing devices may be extinct (i.e. the mixture generated by them does not burn) whereas other may not.
In addition, in the worst cases, operation with fuel at reduced reactivity conditions may also lead to flame extinction (lean blow off or lean blow out, in short LBO).
It is therefore greatly important to ascertain when LBO is approaching, such that countermeasures can be carried out before the flame extinguishes.
FIG. 3 shows a traditional control system of a traditional gas turbine engine 1.
FIG. 3 shows a plenum 2 containing a combustion chamber 3 having a mixing device 4 and a combustion device 5.
The engine 1 has a control system with a pressure sensor 6 detecting the pressure within the combustion device 5 and a further pressure sensor 7 detecting the pressure within the plenum 2 (since the cross sections are very large and the flow velocities are consequently low, the pressure within the combustion device 5 and plenum 2 substantially corresponds to the static pressure).
The sensors 6, 7 are connected to a control unit 8 that drives the engine 1 on the basis of the relationship plotted in FIG. 5.
FIG. 5 shows the function ζ (it is a function of the pressure difference Δp measured through the sensors 6 and 7).
Typically the engine 1 is operated in zone R; in case of lean operation (part load, operation with flue gas recirculation, etc) the operating point may move into zone L.
As shown in FIG. 5, the curve describing the relationship between ζ and the reactivity is flat in zone L (it is also flat at the other side of zone R).
For this reason, when the LBO approaches, ζ remains substantially constant until the LBO is reached and the flame extinguishes; therefore ζ cannot be used to drive the engine operation in the zone L keeping the operating point at a safe distance from the LBO.
In addition, even if when LBO approaches usually CO and UHC (Unburnt Hydro Carbons) emissions strongly increase, the flame shifts downstream (toward the combustion device outlet) and strong low frequency pulsations appear, none of these indicators can be directly connected to the LBO, in other words there is no value of CO or UHC, flame shifting or low frequency pulsations that can indicate that LBO (and thus flame extinction) is imminent.
When the engine is operated with flue gas recirculation the situation is even worse, since typically before the LBO is reached and the flame is extinct no dramatic change in CO, UHT or pulsation is experienced.